As we near completion of our '03 Chevy Silverado project, we have just a few more items to cover to make the truck function as great as it looks. With the addition of a 7-inch Bulletproof/Fox/Atlas lift and 35-inch General ATX tires, the most important remaining mod for this build was to regear the rear end. When GM originally built this truck, the rearend gear ratio was selected by taking into account the available horsepower and torque curve, transmission gearing, and factory tire size, then deciding what would provide the best highway mileage while having power to pull off the line. So when we add lifts and big tires, we throw all that off, adding extra stress on the engine and transmission. Because the first way many of us try to compensate for the loss of power with big tires is with a heavy foot! Another way people try to fix the problem is bolt-on horsepower. Intakes, tuners, headers, and exhaust are great and will help mask the fact that you need gears, but you are not solving the problem.
There seems to be some big mystery surrounding rear ends and gear swaps. Now, you do need some specialized tools and you need to press some parts in and out, but you don't need to be a scientist to set up a differential with new gears—or to pick the correct gear ratio in the first place. While there is an equation for determining the ideal gear ratio for your application, the pros have already figured it out for you. Most of the modern automatics have a reasonably similar high gear, and there are only so many available combinations of ring and pinion combinations, so for most of the trucks we're working on, it goes as follows. If we settled on 33-inch tires, the recommended gear ratio was 4.10:1; 35-inch tires: 4.56:1 gears; 37s: 4.88:1. Of course, if your truck only sees dirt or only does highway driving, you may have different ideas, but these numbers will most often get your engine and transmission working similar to when you had stock tires.
Our '03 Silverado work truck came with a 3.42:1 ratio from the factory and tires around 29 to 30 inches. We initially had the idea to find a limited-slip 2500 rear end from an '11-and-up GM HD truck, but Carl Montoya from Nitro Gear & Axle urged us to stick with our stock 10-bolt. Although the HD rear end would be bulletproof in this truck, we would also be dragging around more than a couple hundred pounds of extra baggage—even when we didn't need the added strength. Carl set us up with the parts to give us a limited-slip rear end, the 4.56:1 ring and pinion we needed, and all the new bearing sets and other parts recommended to do the job right. We even picked up a custom Nitro finned aluminum diff cover and Nitro's own gear oil to finish the job.
Carl also mentioned a name that might help with the install that was a blast from the past—Martin Barraza has been a mobile gear installer for many years around town. He actually did our C10 rearend for us many moons ago, but we had since lost touch. As luck would have it, he still spends a couple days a week at Rebel Off Road in Laguna Hills, California, regearing all the insane Jeep builds done over there, so we made an appointment with him. To say Martin and his assistant are quick would be a gross misrepresentation—we could barely keep up with the camera. And at times when we might accuse a rookie installer of cutting corners, Martin knocks out steps like setting pinion depth, bearing preload, and setting the backlash in seconds like it isn't a concern how the parts go in, but the truth is he has probably done more custom gearsets than anyone else around—ever! He will tell you by feel exactly where the specs are, and when we secretly checked his work he was spot-on! He makes it look so easy we were considering doing the next one ourselves but we'll probably call Martin. Follow along as we rip through the install, do a little cleanup, install the Hellwig rear sway bar, and test to see what the rpm looks like at highway speeds.